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Optical fiber ppt

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Fibre Optics

2. Basic principle Total Internal Reflection in Fiber An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. Light is kept in the "core" of the optical fiber by total internal reflection. 3. 3 What Makes The Light Stay in Fiber Refraction The light waves spread out along its beam. Speed of light depend on the material used called refractive index. Speed of light in the material = speed of light in the free space/refractive index Lower refractive index higher speed 4. 4 The Light is Refracted This end travels further than the other hand Lower Refractive index Region Higher Refractive index Region 5. 5 Refraction When a light ray encounters a boundary separating two different media, part of the ray is reflected back into the first medium and the remainder is bent (or refracted) as it enters the second material. (Light entering an optical fiber bends in towards the center of the fiber refraction) Refraction LED or LASER Source 6. 6 Reflection Light inside an optical fiber bounces off the cladding - reflection Reflection LED or LASER Source 7. 8 Critical Angle If light inside an optical fiber strikes the cladding too steeply, the light refracts into the cladding - determined by the critical angle. (There will come a time when, eventually, the angle of refraction reaches 90o and the light is refracted along the boundary between the two materials. The angle of incidence which results in this effect is called the critical angle). Critical Angle n1Sin X=n2Sin90o 8. 9 Angle of Incidence Also incident angle Measured from perpendicular Exercise: Mark two more incident angles Incident Angles 9. 10 Angle of Reflection Also reflection angle Measured from perpendicular Exercise: Mark the other reflection angle Reflection Angle 10. 11 Reflection Thus light is perfectly reflected at an interface between two materials of different refractive index if: The light is incident on the interface from the side of higher refractive index. The angle is greater than a specific value called the critical angle. 11. 12 Angle of Refraction Also refraction angle Measured from perpendicular Exercise: Mark the other refraction angle Refraction Angle 12. 13 Angle Summary Refraction Angle Three important angles The reflection angle always equals the incident angle Reflection Angle Incident Angles 13. 14 Refractive Index n = c/v c = velocity of light in a vacuum v = velocity of light in a specific medium light bends as it passes from one medium to another with a different index of refraction air, n is about 1 glass, n is about 1.4 Light bends in towards normal - lower n to higher n Light bends away from normal - higher n to lower n 14. 15 Snells Law The amount light is bent by refraction is given by Snells Law: n1sin1 = n2sin2 Light is always refracted into a fiber (although there will be a certain amount of Fresnel reflection) Light can either bounce off the cladding (TIR) or refract into the cladding 15. 16 Snells Law Normal Incidence Angle(1) Refraction Angle(2) Lower Refractive index(n2) Higher Refractive index(n1)Ray of light 16. 17 Critical Angle Calculation The angle of incidence that produces an angle of refraction of 90 is the critical angle n1sin(qc) = n2sin(90) n1sin(qc) = n2 qc = sin-1 (n2/n1) Light at incident angles greater than the critical angle will reflect back into the core Critical Angle, c n1 = Refractive index of the core n2 = Refractive index of the cladding 17. OPTICAL FIBER CONSTRUCTION Core thin glass center of the fiber where light travels. Cladding outer optical material surrounding the core Buffer Coating plastic coating that protect the fiber. 18. OPTICAL FIBER The core, and the lower-refractive-index cladding, are typically made of high-quality silica glass, though they can both be made of plastic as well. 19. 20 NA & ACCEPTANCE ANGLE DERIVATION In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. optical fiber will only propagate light that enters the fiber within a certain cone, known as the acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle, max. 20. 21 When a light ray is incident from a medium of refractive index n to the core of index n1 , Snell's law at medium-core interface gives 21. Substituting for sin r in Snell's law we get: By squaring both sides Thus, 22 22. from where the formula given above follows. NUMERICAL APERATURE IS ACCEPTANCE ANGLE max = 23 23. Definition:- Acceptance angle:- Acceptance angle is defined as the maximum angle of incidence at the interface of air medium and core medium for which the light ray enters into the core and travels along the interface of core and cladding. Acceptance Cone:- There is an imaginary cone of acceptance with an angle .The light that enters the fiber at angles within the acceptance cone are guided down the fiber core Numerical aperture:- Numerical aperture is defined as the light gathering capacity of an optical fiber and it is directly proportional to the acceptance angle. 24 24. 25 Classification of Optical Fiber 25. 26 Three common type of fiber in terms of the material used: Glass core with glass cladding all glass or silica fiber Glass core with plastic cladding plastic cladded/coated silica (PCS) Plastic core with plastic cladding all plastic or polymer fiber 26. Plastic and Silica Fibers 27. BASED ON MODE OF PROPAGATION Two main categories of optical fiber used in fiber optic communications are multi-mode optical fiber single-mode optical fiber. 28 28. Single-mode fiber Carries light pulses along single path Multimode fiber Many pulses of light generated by LED travel at different angles 29 29. Based on the index profile 30 The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded- index fiber 30. 31 Step Index Fibers A step-index fiber has a central core with a uniform refractive index. An outside cladding that also has a uniform refractive index surrounds the core; however, the refractive index of the cladding is less than that of the central core. The refractive index profile may be defined as n(r) = n1 r < a (core) n2 r a (cladding) 31. GRADED-INDEX In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. 32 32. 33 Figure.2.6 (a) (b) 33. 34 multimode step-index fiber the reflective walls of the fiber move the light pulses to the receiver multimode graded-index fiber acts to refract the light toward the center of the fiber by variations in the density single mode fiber the light is guided down the center of an extremely narrow core 34. Figure 2.10 Two types of fiber: (Top) step index fiber; (Bottom) Graded index fiber 35. Attenuation Definition: a loss of signal strength in a lightwave, electrical or radio signal usually related to the distance the signal must travel. Attenuation is caused by: Absorption Scattering Radiative loss 36 36. Losses Losses in optical fiber result from attenuation in the material itself and from scattering, which causes some light to strike the cladding at less than the critical angle Bending the optical fiber too sharply can also cause losses by causing some of the light to meet the cladding at less than the critical angle Losses vary greatly depending upon the type of fiber Plastic fiber may have losses of several hundred dB per kilometer Graded-index multimode glass fiber has a loss of about 24 dB per kilometer Single-mode fiber has a loss of 0.4 dB/km or less 37 37. Macrobending Loss: The curvature of the bend is much larger than fiber diameter. Lightwave suffers sever loss due to radiation of the evanescent field in the cladding region. As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius. For any radius a bit smaller than this point, the losses suddenly becomes extremely large. Higher order modes radiate away faster than lower order modes. 38 38. Microbending Loss Microbending Loss: microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. The power is dissipated through the microbended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber. 39 39. Dispersion The phenomenon in an optical fibre whereby light photons arrive at a distant point in different phase than they entered the fibre. Dispersion causes receive signal distortion that ultimately limits the bandwidth and usable length of the fiBer cable The two main causes of dispersion are: Material (Chromatic) dispersion Waveguide dispersion Intermodal delay (in multimode fibres) 40 40. Dispersion in fiber optics results from the fact that in multimode propagation, the signal travels faster in some modes than it would in others Single-mode fibers are relatively free from dispersion except for intramodal dispersion Graded-index fibers reduce dispersion by taking advantage of higher-order modes One form of intramodal dispersion is called material dispersion because it depends upon the material of the core Another form of dispersion is called waveguide dispersion Dispersion increases with the bandwidth of the light source 41 41. Advantages of Optical Fibre Thinner Less Expensive Higher Carrying Capacity Less Signal Degradation& Digital Signals Light Signals Non-Flammable Light Weight 42. Advantages of fiber optics Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber Immunity to Noise - Immune to electromagnetic interference (EMI). Safety - Doesnt transmit electrical signals, making it safe in environments like a gas pipeline. High Security - Impossible to tap into. 43. Advantages of fiber optics Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation) Reliability - More resilient than copper in extreme environmental conditions. Size - Lighter and more compact than copper. Flexibility - Unlike impure, brittle glass, fiber is physically very flexible. 44. Fiber Optic Advantages greater capacity (bandwidth up to 2 Gbps, or more) smaller size and lighter weight lower attenuation immunity to environmental interference highly secure due to tap difficulty and lack of signal radiation 45. Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult. Disadvantages of fiber optics 46. Areas of Application Telecommunications Local Area Networks Cable TV CCTV Optical Fiber Sensors 47. 48 Formula Summary Index of Refraction Snells Law Critical Angle Acceptance Angle Numerical Aperture v c n = 2211 sinsin nn = = 1 21 sin n n c ( )2 2 2 1 1 sin nn = 2 2 2 1sin nnNA == 48. STUDENTS YOU CAN ALSO REFER IT 49 http://hank.uoregon.edu/experiments/Dispersion-in- Optical-Fiber/Unit_1.6%20(2).pdf http://www1.ceit.es/asignaturas/comuopticas/pdf/chapter4. pdf http://course.ee.ust.hk/elec342/notes/Lecture %206_attenuation%20and%20dispersion.pdf 1 Engineering Physics by H Aruldhas, PHI India 2 Engineering Physics by B K Pandey , S. Chaturvedi, Cengage Learning 3Resnick, Halliday and Krane, Physics part I and II, 5th Edition John Wiely 4Engineering Physics by S.CHAND 5Engineering Physics by G VIJIYAKUMARI

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